This invention relates to an angularly rotatable mirror apparatus adapted for use with a device for effecting slit exposing of a photosensitive member to an optical image of an original to be copied by a copying machine.
In one type of angularly rotatable mirror apparatus known in the art of electronic copying, an angularly rotatable mirror is adapted to reflect light, which emanates from a light source of an original exposing device, toward an original placed on an original supporting glass plate so as to scan the surface of the original by the reflected light at a constant speed while the mirror is angularly rotated. In this type of angularly rotatable mirror apparatus, it is necessary that the light reflected by the angularly rotated mirror scan the surface of the original at a constant speed so as to expose a photosensitive member of a copying machine to an accurate optical image of the original. To attain this end, the angular velocity of the angularly rotated mirror should be varied in accordance with a change in the position of the original which is scanned by the reflected light. Stated differently, the mirror should not be angularly rotated at a constant angular velocity.
In one known mechanism for driving an angularly rotatable mirror to cause the same to angularly rotate at angular velocities which vary so as to meet the aforesaid requirement, a servomotor 3 is directly connected to a rotary shaft 2 for supporting the mirror 1 which shaft supports thereon an angle detector 4 and an angular velocity detector 5, as shown in FIG. 1(a). Detection signals supplied by the two detectors 4 and 5, as well as a required angle signal A and a required angular velocity signal B, are fed to a control circuit 6 which controls the servomotor 3. If the motor is an AC motor, this mechanism is not suitable for controlling a miniscule angle, because the number of poles is finite and the phase control of the voltage of a control winding is a complicated process.
As shown in FIG. 2, an actual path P of angular movement of the mirror 1 as plotted in a diagram of rectangular coordinates in which the abscissa indicates the time and the ordinate represents the angle of rotation tends to deviate from a required path P.sub.0 and show drift. In case the motor is a DC motor, the number of poles of the motor stator affects the actual path of angular movement of the mirror 1 and gives rise to a cocking phenomenon, with the result that the actual path also deviates from the required path, showing drift.
FIG. 1(b) shows another mechanism for driving the mirror 1 to cause the same to angularly rotate, in which the driving force of the servomotor 3 is directly transmitted to the mirror 1 through a belt 7 or a gear, not shown. In this mechanism, deviation of the actual path from the required path or drift, which is caused by the number of poles of the motor stator, is minimized. However, wobbling of the belt 7 or gear (due to backlash, mechanical vibration, etc. ) tends to cause deviation of the angular movement from the required path.
A still another mechanism shown in FIG. 1(c) is known in which a constant function is given to the angular rotation of the mirror 1 by using a cam 8 and a link 9. In this mechanism, difficulties are encountered in eliminating influences which are exerted on the angular rotation of the mirror by the wobbling of the cam, the degree of precision with which the cam is finished, and the presence of dust on the surface of the cam.